Study of tau neutrino production with nuclear emulsion at CERN-SPS

The tau neutrino interaction cross-section was directly measured in the DONuT experiment but it suffers from a large systematic (∼50%) and statistical (∼30%) errors. The main source of the systematic error was due to poor knowledge of the tau neutrino flux. The DsTau experiment at CERN-SPS has been proposed to measure an inclusive differential cross-section of the Ds production with a consecutive decay to tau lepton in p-A interactions, which is a main source of tau neutrinos in neutrino beams at accelerators. The search for Ds → τ → X decay is challenging due to milimetric range of Ds and τ decay lengths and a small angle between them of the order of few mrad. Nevertheless a precise measurement is possible with the use of nuclear emulsion providing a sub-micron spatial resolution. The experiment is aiming to collect 2.3 × 10$^{8}$ proton interactions in the tungsten target, and to detect ∼ 1000 Ds → τ → X decays. DsTau successfully performed a pilot run in 2018 collecting ∼10% of the full data and was approved as NA65 in 2019. The results from the pilot run are presented in this paper and the prospect for physics runs in 2021–2022 are given.